Electrical currents and voltages were measured on wheel bearings of heavy loaded ore trains in the north of Sweden, while the geographical coordinates and the velocity of the train were recorded by means of GPS. The RMS-magnitudes and the frequency spectra of the currents have been recorded and compared for different positions on the train, and at different driving conditions, including very low winter temperatures. The results showed substantial electrical currents going through the wheel bearings and much higher currents in modern trains with 30 tons axle load compared to old trains with 25 tons axle load. The measured voltage over the wheel bearings showed that the voltage varied depending on the axle load.